Digital Control of Power Converters

ABSTRACT

A system and method for controlling a power converter is presented. An embodiment comprises an analog differential circuit connected to an analog-to-digital converter, a digital pulse generator, and a pre-driver to control the power converter. Another embodiment also includes a digital filter as part of the control loop that may be used to control the loop characteristics of the control circuit. Yet another embodiment replaces the differential circuit with a sigma-delta analog-to-digital modulator and a decimator.

This application claims the benefit of U.S. Provisional Application No. 61/038,348, filed on Mar. 20, 2008, entitled “Digital Control of Power Converters,” which application is hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to a system and method for power conversion, and more particularly to a system and method for controlling the output voltage of DC/DC converters.

BACKGROUND

Generally, today's devices have systems with multiple power requirements, but which run off of a smaller number of, or even a single, power supplies, such as batteries. This is especially true of system-on-chip (SOC) devices, which contain multiple sections with different power requirements, but contain few connections to different power supplies. This is also true of portable devices that run off a single battery (such as a cellular phone). These devices and SOCs utilize power converters to take a single power source and convert the power to the levels necessary for each section.

FIG. 1 illustrates a typical power converter 101 along with its associated control circuitry 103. The power converter 101 typically comprises a Power P-FET 105 and a Power N-FET 107 in series between a power source 109 and a ground 111. A capacitor 117 runs in parallel with a power load 115, and an inductor 113 is connected between the capacitor 117/power load 115 and the connection between the P-FET 105 and the N-FET 107.

During operation of the power converter 101 the inductor 113 is connected to the power source 109 through the P-FET 105, which charges the inductor 113 during an ON phase, storing energy. When the inductor 113 is charged, the inductor 113 discharges the stored energy as an output voltage V_(Out) to the power load 115. The control circuitry 103 controls the “on” and “off” states of the P-FET 105 and the N-FET 107 such that the desired output voltage V_(Out) is supplied to the power load 115.

The traditional control circuitry 103 consists of a continuous voltage, discrete time analog circuit comprising a first op amp 119, also called error amp, a comparator 121, a ramp generator 123, and a pre-driver 125. The positive input to the first op amp 119 is connected to the power converter 101 between the inductor 113 and the capacitor 117/power load 115, while the negative input to the first op amp 119 is connected to a reference voltage V_(Ref). The output of the first op amp 119 is connected to the negative input of the second op amp 121, while the output of the ramp generator 123 is connected to the positive input of the second op amp 121. The output signal from the second op amp 121 is routed to the pre-driver 125, which buffers the signals to the P-FET 105 and the N-FET 107, effectively controlling the power converter 101 in either “on” or “off” mode.

One disadvantage of the analog control circuit 103 is the difficulty in implementing a precise ramp generator 123. Further, it is difficult to control the loop response and dynamic performance of the control circuit 103 and power converter 101 using an analog system because the frequency response of the control loop can only be modified by changing the frequency response of the analog components, requiring a redesign and rebuild. In particular, error amplifier 119 must typically be designed with a certain frequency response in order to ensure the proper control loop dynamic behavior and stability.

Accordingly, what is needed is a control circuit that does not require a ramp generator and that allows easier control of the loop response of the control circuit. It is also desirable to have a control circuit that can be implemented using a low supply voltage, as is often required in advanced semiconductor processes.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present invention which provide for a control circuit for a power converter.

In accordance with a preferred embodiment of the present invention, a system for controlling a power converter comprises a differential circuit configured to compare an output voltage to a reference voltage to produce an error signal. An analog-to-digital converter takes the error signal and converts it to a digital signal. A digitally controlled PWM generator receives the digital signal and generates control pulses to control transistors in a power converter, thereby controlling the output voltage.

In accordance with another preferred embodiment of the present invention, a system for power conversion comprises a power converter with first and second transistors. A control circuit is configured to control the first and second transistors with a differential circuit, an analog-to-digital converter, and a digitally controlled PWM generator similar to the previous embodiment. However, a digital filter is also included as part of the control circuit to filter the digital signal.

In accordance with yet another preferred embodiment of the present invention, a power conversion system comprises a power converter configured to operate at a first clock frequency. An analog-to-digital converter is configured to sample the output voltage from the power converter at a second clock frequency higher than the first clock frequency and then to send the signal to a digital filter and a digital pulse generator to generate a pulse to control the power converter.

An advantage of a preferred embodiment of the present invention is a much greater ability to control the loop dynamics of the system. By implementing a portion of the system digitally, these loop dynamics can be precisely controlled, or even changed, by simply reprogramming the system, as opposed to having to redesign and rebuild the entire system.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a prior art circuit diagram of a power converter and its associated analog control circuit;

FIG. 2 illustrates a power converter and its associated partially digital control circuit in accordance with an embodiment of the present invention;

FIG. 3 illustrates a voltage and current diagram for pulse width modulated operation in accordance with an embodiment of the present invention;

FIG. 4 illustrates a voltage and current diagram for auto mode operation in accordance with an embodiment of the present invention;

FIG. 5 illustrates a voltage and current diagram for pulse frequency modulation operation in accordance with an embodiment of the present invention;

FIG. 6 illustrates a power converter and its associated partially digital, filtered control circuit in accordance with an embodiment of the present invention; and

FIG. 7 illustrates a power converter and its associated oversampled, partially digital control circuit in accordance with an embodiment of the present invention.

Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.

The present invention will be described with respect to preferred embodiments in a specific context, namely a control circuit for a DC-to-DC power converter. The invention may also be applied, however, to other control circuits.

With reference now to FIG. 2, there is shown a power converter 201 and its associated control circuit 203. The power converter 201 is preferably a DC-to-DC “Buck” type power converter that supplies an output voltage V_(Out) to a load 215. The power converter 201 preferably comprises a Power P-FET 205 connected in series to a Power N-FET 207, with the gates of the P-FET 205 and the N-FET 207 controlled by the control circuit 203. The P-FET 205 is preferably connected to an input voltage source 209, while the N-FET 207 is preferably connected to a ground 211. The shared connection between the P-FET 205 and the N-FET 207 is preferably connected through an inductor 213 to the load 215, and a capacitor 217 is preferably connected in parallel with the load 215 to provide smoothing of the output voltage V_(Out).

While the precise parameters of the power converter 201 are, of course, dependent upon the required load and design, one preferred embodiment of the power converter 201 preferably has an inductor 213 that has an inductance of between about 1 μH and about 10 μH, with a preferred inductance of about 2.2 μH. Further, the capacitor 217 preferably has a capacitance of between about 1 μF and about 20 μF, with a preferred capacitance of about 10 μF, and the load 215 preferably requires a current of between about 1 mA and about 1,000 mA, with a preferred inductance of about 600 mA.

It should be recognized that the preferred “Buck” type of power converter 201 is merely representative of one possible configuration for a power converter 201 that may be utilized with the present invention. Other configurations and designs for the power converter 201 may also be used with the present invention, and these configurations and designs are fully intended to be included within the scope of the present invention.

The control circuit 203 (whose design is more fully described below) controls the P-FET 205 and the N-FET 207 to regulate the output voltage V_(Out) coupled to the load 215. The control circuit 203 turns the P-FET 205 and the N-FET 207 “On” and “Off” in an alternating manner to charge and discharge the inductor 213, which supplies the current through the load 215. By controlling the timing and duration of the charging and discharging of the inductor 213, the output voltage V_(Out) may be controlled.

The control circuit 203 preferably controls the P-FET 205 and the N-FET 207 in one of three manners. In Pulse Width Modulation mode (PWM mode) the control circuit 203 alternatively connects the inductor 213 to either the input voltage source 209 or the ground 211. FIG. 3 illustrates the voltage pulse and current used to charge the inductor 213. As illustrated, the width of the voltage pulse is variable (as illustrated by the dotted line) depending upon the instantaneous voltage output V_(Out), and the current oscillates around a desired value, sometimes even going negative (e.g., the current is flowing into the power converter 201 instead of out of the power converter 201) at low currents. PWM mode may be desirable for use with high currents, such as currents greater than one-third of the maximum current of the power converter 201 (which is dependent upon the design of the power converter 201).

FIG. 4 illustrates a second preferred mode of operation, Auto mode. The Auto mode of operation is similar to the PWM mode except that, in addition to alternating between being connected to the input voltage source 209 and the ground 211, both the P-FET 205 and the N-FET 207 may be turned to an “Off” state during part of the time period. This effectively prevents the current from going negative, as is possible in PWM mode. As such, at high currents Auto mode acts similarly to PWM mode, but at low currents (e.g., currents less than ⅓ of the maximum current of the power converter 201), Auto mode creates a variable pulse and prevents the current from going negative. Because of this, Auto mode is usually the preferred default mode.

FIG. 5 illustrates a third preferred mode of operation, Pulse Frequency Modulation (PFM) mode, also called Discontinuous Current Mode (DCM). In PFM mode a small voltage pulse is generated when the output voltage V_(Out) drops below a lower threshold, and each of these pulses adds a small charge to the inductor 213, whose voltage output V_(Out) decays in between the pulses. Each of the generated voltage pulses has the same time and shape, and the current is determined by the number of voltage pulses, not the duration of the pulses, as in PWM mode.

Because of this, PFM mode has historically been controlled using a hysterectic type of control implemented with two analog comparators to establish an upper and lower limit for the output voltage V_(Out). For this control scheme, a pulse is generated if V_(Out) is below the lower threshold, and a pulse is not generated if V_(Out) is over the upper threshold. This results in a “pulse train” that causes more ripple on the output voltage V_(Out). PFM mode is preferably used in low current operations, such as “sleep” or “standby” modes.

Returning to FIG. 2, the present invention implements these modes of operation using a preferred embodiment comprising a differential amplifier 219 to compare the output voltage V_(Out) to a reference voltage V_(Ref) and an analog-to-digital converter (ADC) 221 to convert the analog signal into a digital signal. A digitally controlled PWM generator 223 and a pre-driver 225 are preferably used to send pulses and to control P-FET 205 and N-FET 207. This preferred embodiment allows for a digital approach of controlling the power converter 201 instead of the traditional analog approach and its inherent drawbacks.

Preferably, the differential amplifier 219 comprises an error amplifier, although other configurations that produce an error signal may alternatively be used. The differential amplifier 219 amplifies the voltage difference between a non-inverting (+) input and an inverting (−) input. The reference voltage V_(Ref) chosen by the design requirements is connected to the inverting (−) input and the voltage output V_(Out) from the power converter 201 is connected to the non-inverting (+) input. In this configuration differential amplifier 219 compares V_(Out) to V_(Ref), and amplifies the difference to an output error signal V_(err). A scaled version of V_(Out) may also be used to compare against V_(Ref). In that case, the output voltage will become a multiplied version of V_(Ref). It should be noted that because the error signal V_(err) is normally very small (especially when the control loop locks), the differential input range of the error amplifier can be very limited, typically to within a few tens of mV.

The analog output error signal V_(err) from the differential amplifier 219 is connected to an input of the ADC 221. By converting only the error signal V_(err) between V_(Out) and V_(Ref) instead of the entire V_(Out) signal, the effective range of the control circuit 203 can be increased since the error signal V_(err) will remain relatively small no matter what the actual V_(Out) may be. This allows for a much larger range of control without the fear of overloading the control circuit 203, and also makes the task of designing the analog components of control circuit 203 much easier.

The ADC 221 converts the analog output error signal V_(err) from differential amplifier 219 into an N-bit digital signal D_(out). Analog output error signal V_(err) is first sampled into a discrete-time signal and then the discrete-time signal is quantized into a finite number of quantization levels to produce D_(out). For an N-bit digital signal D_(out), the error signal V_(err) is quantized into 2N levels, with each level separated by a quantization step size.

The ADC 221 is preferably a Sigma-Delta ADC, although other types of ADCs, such as a flash ADC, a Pipeline ADC, Successive-Approximation ADC, Integrating ADC, or a Delta-Encoded ADC, could alternatively be used. ADC 221 preferably has a low-resolution and high-gain that produces an output with an accuracy of between 2 bits and 8 bits, with an even more preferred output of 5 bits. The ADC 221 is preferably over-sampled (as discussed below with reference to FIG. 7), but may alternatively operate as a Nyquist ADC where the sampling rate is equal to the clock/switching frequency of the P-FET 205 and the N-FET 207.

The digital signal D_(out) of the ADC 221 is connected to the input of the digitally controlled PWM generator 223. Preferably, the digitally controlled PWM generator 223 employs a counter (not shown) which counts up to the value converted by the ADC 221. However, other types of digitally controlled PWM generators 223, such as digital one-shot or various digitally controlled timers, may alternatively be utilized. For a counting-type digitally controlled PWM generator 223, the counting frequency is preferably a multiple of the switching frequency of the P-FET 205 and the N-FET 207 so that the duty cycle will be proportional to the output of the ADC 221.

The output signal P_(Out) of the digitally controlled PWM generator 223 is routed to the input of a Pre-Driver 225. The Pre-Driver 225 is preferably designed to amplify the output signal P_(Out) from the digitally controlled PWM generator 223 and to control the P-FET 205 and the N-FET 207 in order to generate the pulses to the inductor 213. Additionally, the Pre-Driver 225 is preferably designed to control the P-FET 205 and N-FET 207 in such a way as to prevent shoot-through current from the input voltage source 209 to the ground 211. As such, any number of Pre-Drivers 225 may be used to control the P-FET 205 and the N-FET 207, and any of these Pre-Drivers 225 may be used with the present invention.

FIG. 6 illustrates a second preferred embodiment in which a programmable digital filter 601 is connected between the ADC 221 and the digitally controlled PWM generator 223 of FIG. 2. The digital filter 601 is preferably programmable to supplement and better control the analog frequency response and transfer function so that the loop response of the system can be precisely controlled. In other words, the frequency response of the digital filter 601 can be matched to the remaining analog components of the control circuit 203 to achieve any desired loop response, thereby eliminating the problems associated with analog loop control.

Additionally, the digital filter 601 could also be programmed for additional functions besides frequency response control. Functions such as integration, differentiation, supplying additional DC gain, combinations of these functions, or the like could be made by programming such functions into the digital filter 601. Further, reprogramming the digital filter 601 may be performed adaptively and on-line while the power converter 201 and the control circuit 203 are operating, without requiring a complete shutdown of the system for each reprogramming.

The advantages of such a programmable, digital filter 601 are immense. By using a programmable digital filter 601 to complement and control the frequency response of the analog components, the design of the system becomes much more flexible than if each component had to be designed and built for an analog loop response. Further, adjustments could be made post-production, if necessary, by reprogramming the digital filter 601, instead of having to completely redesign and reproduce the entire power converter 201 and control circuit 203.

FIG. 7 illustrates another preferred embodiment of the present invention in which the differential amplifier 219 is combined into an over-sampled ADC 701 and a decimator 703. By using the over-sampled ADC 701, the system may be better able to average the output voltage V_(Out) due to a better capture of transients. Further, a higher clock rate may be implemented in the oversampled ADC 701, which may be more compatible with advanced process technology.

In a preferred embodiment, the over-sampled ADC 701 is implemented by combining the differential amplifier 219 with a Sigma-Delta modulator/ADC 701 in series with a decimator 703. The Sigma-Delta ADC 701 preferably oversamples the output signal V_(Out) by sampling at a rate that is much higher than the Nyquist frequency to produce the digital output D_(Out), thereby concentrating quantization noise in the higher frequencies. The Sigma-Delta ADC 701 preferably has an oversampling frequency that is a multiple of the Nyquist frequency, and preferably has an oversampling ratio of between about 2 and about 256, with a preferred oversampling ratio of about 32.

The digital output signal D_(Out) from the Sigma-Delta ADC 701 is connected to the input of the decimator 703. The decimator 703 preferably performs two separate functions. First, the decimator 703 is preferably designed to resample the digital output signal D_(Out) from the Sigma-Delta ADC 701 and provide a multi-bit data word at the Nyquist rate. Additionally, the decimator 703 is also designed as a low-pass filter to remove the quantization noise that has been concentrated in the higher frequencies, thereby increasing the resolution of the Sigma-Delta ADC 701. A simple way of implementing the decimator for a 1-bit sigma-delta converter is to implement a circuit counting the number of logic ones coming out of the converter during one period of the lower frequency clock (Nyquist rate).

These preferred embodiments of the present invention provide great flexibility and adaptability to the design and operation of power converter control circuits. By implementing a portion of the system digitally, control of the overall loop dynamics of the entire system can be modified as needed without completely redesigning and rebuilding the system, with the adjustments even capable of being made while the circuit is in operation. Further, by utilizing an oversampled ADC, the system may better capture transients and better average the output voltage of the power converter.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. For example, it will be readily understood by those skilled in the art that the type of power converter may be varied while still remaining within the scope of the present invention.

Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

1. A system for controlling a power converter, the system comprising: a differential unit with an input connected to a voltage to be controlled from the power converter; an analog-to-digital converter communicably coupled to an output of the differential unit; a digital filter communicably coupled to an output of the analog-to-digital filter; and a digital pulse generator communicably coupled to an output of the digital filter.
 2. The system of claim 1, wherein the digital filter is configured to have an output of between about 1 to about 8 bits.
 3. The system of claim 1, wherein the digital filter is configured to control a loop response of the system.
 4. The system of claim 1, wherein the analog-to-digital converter comprises a sigma-delta modulator.
 5. The system of claim 4, further comprising a decimator and a digital filter communicably coupled between the sigma-delta modulator and the digital pulse generator.
 6. The system of claim 1, wherein the differential unit is configured to generate an error signal.
 7. The system of claim 6, wherein the analog-to-digital converter is configured to oversample the error signal.
 8. The system of claim 1, further comprising a pre-driver communicably coupled to the output of the digital pulse generator.
 9. The system of claim 7, further comprising a power converter communicably coupled to the input of the differential circuit, and communicably coupled to the output of the pre-driver.
 10. A system for power conversion control, the system comprising: a differential circuit comprising a first input connected to a reference voltage, a second input connected to a voltage to be controlled, and an error output; an analog-to-digital converter communicably coupled to the error output of the differential circuit; and a digital pulse generator communicably coupled to an output of the analog-to-digital converter.
 11. The system of claim 10, further comprising a digital filter communicably coupled between the analog-to-digital converter and the digital pulse generator.
 12. The system of claim 11, wherein the digital filter is configured to control a loop response of the system.
 13. The system of claim 10, wherein the analog-to-digital converter is configured to oversample an analog error signal from the differential circuit.
 14. The system of claim 10, further comprising a pre-driver communicably coupled to the output of the digital pulse generator.
 15. A power conversion system comprising: a power converter configured to operate at a first clock frequency; an analog-to-digital converter communicably coupled to an output voltage of the power converter, the analog-to-digital converter configured to sample the output voltage at a second clock frequency higher than the first clock frequency; a first digital filter communicably coupled to the analog-to-digital converter, the first digital filter configured to output a signal at the first clock frequency; and a digital pulse generator communicably coupled to the first digital filter and the power converter.
 16. The power conversion system of claim 15, wherein the analog-to-digital converter comprises a sigma-delta modulator.
 17. The power conversion system of claim 16, further comprising a differential circuit communicably coupled between the power converter and the analog-to-digital converter, the differential circuit configured to output an error signal to the analog-to-digital converter.
 18. The power conversion system of claim 15, wherein the second clock frequency is a multiple of the first clock frequency.
 19. The power conversion system of claim 15, wherein the digital pulse generator comprises a counter.
 20. The power conversion system of claim 15, further comprising a second digital filter communicably coupled between the first digital filter and the digital pulse generator. 